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The delta variant of the coronavirus, now the most common strain circulating in the United States, is increasing cases of Covid-19 across the country. The key to the dominance of the variant is its supercharged transmissibility, due in part to the behavior of the mutated virus in the body after infection.
The delta variant is considered the most contagious of all the variants known to date, with research suggesting that it is more than twice as transmissible as the original strain of the coronavirus. What sets Delta apart is the amount of virus produced by those infected – a measurement known as viral load, experts say.
“We have seen through testing that people infected with the delta variant have more virus in their samples than people who have another subtype of the virus,” said Susan Butler-Wu, clinical microbiologist at the University. from Southern California. “That’s a big part of what will explain this improved transmissibility.”
Viral load is a term that refers to the amount of virus present in the body. A study by Chinese researchers that has not yet been peer reviewed found that people infected with the delta variant had viral loads up to 1,260 times higher than the viral loads of people infected with the strain original that circulated in 2020.
Research is ongoing, but the results, which were posted on July 12 on the MedRxiv preprint server, help explain why the delta variant has been linked to large clusters of infection and how it has spread so quickly. in the world.
“Patients with higher viral loads are more likely to pass the virus on to other people,” said Dr. Randall Olsen, medical director of the Molecular Diagnostic Laboratory at Houston Methodist Hospital. “In addition, the higher the viral load, the sicker the patient is generally. “
The significantly higher viral loads seen by Chinese scientists appear to support other research that has focused on the increased transmissibility of the delta variant. Public health officials in the UK, where the delta variant fueled a new wave of infections in early June, said the variant could be 40-60% more contagious than the alpha variant, which was previously the dominant strain .
One way to estimate viral load through testing is to look at what are known as cycle cutoff values, or Ct. The most accurate tests are to take a sample from the nose or upper part of the body. throat with a swab. The tests use a method called a polymerase chain reaction, or PCR, to detect genetic material – pieces of viral RNA – that are present in the body when the virus is active.
In a laboratory, the collected sample undergoes several cycles of a process called amplification. This involves making millions of copies of a piece of the virus’s genetic material so that even a small amount of the virus can be detected if it is present in the sample. Scientists can repeat this process for up to 40 cycles per test. The Ct value refers to the cycle number where a sample goes from negative to positive.
“It’s an inverse relationship, so a low Ct means there’s a lot of virus and you can get a positive result earlier in the test,” Olsen said.
As such, Ct values have sometimes been used as an indicator of viral load and transmissibility, but Butler-Wu said it was not an accurate gauge.
“There is no infectivity test for Covid,” she said. “The problem is that the discussion around Ct values has been very simplified.”
In an internal document that leaked last week from the Centers for Disease Control and Prevention, the agency presented data, some of which was preliminary, on the delta variant and cases where fully vaccinated individuals were infected. The CDC’s investigation looked at delta infections in vaccinated and unvaccinated people and found that sample Ct values were similar for both groups.
The CDC’s internal presentation concluded that “breakthrough infections can be as transmissible as unvaccinated cases.”
But people shouldn’t infer infectivity from a Ct value, said Dr. Krutika Kuppalli, an infectious disease physician and assistant professor at the Medical University of South Carolina.
“There is a relationship between Ct values and the amount of virus in the sample, but they are not always equivalent,” she said. “In other words, you can correlate a Ct value with viral load, but not always.”
This is because the quality of a sample can affect Ct values, in addition to when a person has been tested during their infection, Butler-Wu said.
“So much varies and can impact a Ct value, so all we can safely say is that this is a snapshot in time,” Butler-Wu said.
While CDC results indicate that vaccinated people infected with the delta variant may be able to spread the virus just as easily as unvaccinated people, vaccines have been shown to protect the vast majority of breakthrough infections against serious illnesses. , hospitalization and death.
But beyond Ct values, studies have found features of delta and the disease it causes that suggest this variant is in a category of its own.
Chinese researchers who studied viral loads in people infected with the delta variant also observed that the incubation period with delta patients was shorter than that of the original strain. With the previous variants, the virus was detectable in infected individuals an average of six days after exposure, but with the delta variant, this window was reduced to four days.
The change in incubation period could indicate that the delta variant is better able to invade cells and can replicate faster than previous variants, which may help the virus to spread. The shorter window also makes contact tracing even more difficult for already overburdened public health departments.
More research is needed, but there are also indications that the delta variant may cause more serious illness. A study published on June 14 in the journal The Lancet looked at the impact of the delta variant in Scotland, where it was the dominant strain. The researchers found that the risk of hospitalization from Covid-19 was roughly doubled for patients infected with the delta, compared to those infected with the alpha variant, which was previously the dominant strain worldwide.
Still, experts say there’s a lot to be learned about the highly contagious delta variant, including how it affects children and whether it causes more serious long-term symptoms. The severity of the outbreaks caused by the delta reinforces the need for people to get vaccinated and double mitigation measures to slow the spread of the virus, Olsen said.
“We have to continue to insist that people have to get vaccinated if they haven’t already,” he said. “This is how we are going to protect ourselves, and this is ultimately how we will come out of this pandemic. “
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